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Bone:Pelvis:Pelvic bone
Bones of the Pelvis The pelvis is a bony ring comprised of paired innominate bones, the sacrum and the coccyx Innominate bones: * Articulate with each other anteriorly and the sacrum posteriorly * Composed of three parts which fuse at the acetabulum: * Ilium ** Flat curved bone ** Iliac crest superiorly with anterior and posterior superior iliac spines on either end of the iliac crest with the anterior and posterior inferior iliac spines below them ** Inner surface is smooth with the iliopectineal line running from front to back – demarcating the true from the false pelvis ** Iliacus muscle arises on its inner surface * Ischium ** Composed of a body and an inferior ramus which joins the inferior pubic ramus ** The body has a tuberosity inferiorly and a spine posteriorly ** The ischial spine defines the greater and lesser sciatic notchs * Pubic bone ** Consists of a body and superior and inferior rami ** The body of the pubic bone articulates with its fellow at the symphysis pubis. The articular surfaces are covered in hyaline cartilage with a fibrocartilaginous disc between them ** Has a tubercle on its superomedial aspect Sacrum: * Triangular in shape * Comprised of five fused vertebrae * The anterior part of the upper end is called the sacral promontory * Articulates with the lumbar spine superiorly and the coccyx inferiorly * Anteriorly the sacrum has four pairs of sacral foramina which transmit nerves from the sacral canal, lateral to these are the lateral masses or sacral ala * There are four pairs of posterior sacral foramina * The sacral canal ends posteriorly in the sacral hiatus (midline opening that transmits the fifth sacral nerves) Coccyx: * Composed of 3-5 fused vertebrae, the first segment is often separate * Articulates at an acute angle with the sacrum Sacroiliac joints: Covered with cartilage and lined with synovium The joint surface is flat and uneven, helping to lock the sacrum into the iliac bones Muscles of the pelvis: * The iliopsoas ** Psoas fuses with iliacus and passes anteriorly, under the inguinal ligament to insert into the lesser trochanter of the femur * The piriformis muscle ** Passes obliquely from the anterior aspect of the sacrum through the greater sciatic foramen to insert into the greater trochanter of the femur * The obturator internus muscle ** Arises from the anterior and lateral walls of the pelvis covering the obturator foramen, its fibres converge towards the lesser sciatic foramen as it hooks around the posterior part of the ischium between its tuberosity and spine, passing through the lesser sciatic foramen and insers into the greater trochanter of the femur * The gluteal muscles ** Arise from the external surface of the iliac bone and the iliac crest and insert into the upper femur ** Gluteus maximus *** Largest and most superficial, covers the posterior part of the ilium and SIJs ** Gluteus medius and minimus *** More anteriorly placed with gluteus minimus the smallest and deepest Ossification: * The pelvic bone is ossified from 8 centres * Primary for each of iliac, ischium and pubis, and * 5 secondary: iliac crest, AIIS, ichial tuberosity, pubic symphysis, and base of acetabulum. * The centres appear during fetal development, but the pelvis remains largely cartilaginous. Timeline: * 7 years, ischial tuberosity and inferior ramus ossify. * 14 years, acetabulum ossifies but Y-shape remains seperated by cartilage. * 20 to 25 years, fully ossified Variants: Differences between the males and female bony pelvis arise as an adaptation of the female pelvis to childbearing: * infrapubic angle is greater than 90 degrees in females * pelvic inlet shape ** males - heart-shaped ** females - round or oval * wider greater sciatic notch in females * acetabulum faces more anteriorly in females * sacrum more triangular and shorter in females * oval obturator foramen in females Non-gender variants: * Conversion of the obturator sulcus (or groove) into a bony foramen has been reported. * Defect or nonunion of the pubic and ischial rami is occasionally observed. * The acetabular notch may be absent. * The dimension of the greater sciatic notch is variable. * An accessory ischial spine may project from the ilium/ischium synchrondrosis, separating the greater sciatic foramen into two compartments. * An accessory spine behind the ileopectineal eminence occasionally provides an attachment site for the psoas minor muscle. * The sacral canal os the continuation of the vertebral canal. The subdural and subarachnoid spaces extend within the canal as far as the middle third of the body of the second sacral vertebra in 46% of 56 cadavers examined